Steam has a CLI that installs games. I’ve been meaning to use it to install each of my games and archive their installation for future offline use.

That’s not as convenient as an easy-to-download offline installer, certainly, but seems workable in theory. Maybe I’m unaware of limitation(s) to this approach?

https://developer.valvesoftware.com/wiki/SteamCMD

SteamCMD doesn't strip Steam DRM - even with files backed up (which the Steam GUI will do, no need for SteamCMD), you wont be able to use them without an internet connection and authenticating with Steam every now and then.

GOG's bundles are offline and DRM free. You could store them on a disk, and 30 years from now reinstall and play on an airgapped computer with no authentication required.